When data is transmitted from a transmitter to a receiver, it is important for the data to reach the receiver with a satisfactory level of quality and also for the data transmission to take up as few resources as possible.
The data is coded to make it less sensitive to errors on the transmission path. With wireless communication networks, the transmitter can also adapt its transmit power to transmission conditions.
In order to be able to code appropriately for the current transmission, the transmitter must know the receive quality at the receiver. The modulation system and code rate can both be adapted in the context of adaptive coding. For mobile communication networks this is also referred to as adaptive modulation and coding (AMC). The modulation system refers to the manner in which the carrier is modified as a function of the signal carrying the information. In mobile radio for example the QPSK (quadrature phase shift keying) modulation system or 16 bit QAM (quadrature amplitude modulation) is used. The code rates indicate how many kilobits for example are transmitted per second (Kbps).
The modulation system is often extended into a modulation and coding system MCS, in which the code rate is indicated as well as the modulation system.
This is considered in more detail below with reference to an example from a specific communication network, a UMTS mobile radio system, for which the following terms are introduced:
Terms Used
A communication system or communication network is a structure for the exchange of data. It can be a cellular mobile radio network, such as the GSM (global system of communications) network or the UMTS (universal mobile telecommunications system) network. Terminals and base stations are generally provided in a communication system. In UMTS, the communication system or radio transmission network has at least base stations, also referred to here as nodes, as well as radio network controllers (RNC) to connect the individual base stations. The terrestrial radio access network, or universal terrestrial radio access network UTRAN, is the radio element of a UMTS network, in which a radio interface is also provided. A radio interface is standardized and defines all the physical and protocol-related specifications for the data exchange, for example the modulation method, the bandwidth, the frequency swing, access methods, safeguarding procedures and even switching methods. UTRAN therefore comprises at least base stations and at least one RNC.
In the case of cellular mobile radio systems, different radio transmission technologies can be provided, which define how the physical connection resources are organized. In the case of UMTS, a frequency division duplex (FDD) mode is currently provided as well as various time division duplex (TDD) modes. Using FDD mode, data is transmitted from so-called uplink and downlink connections on different frequencies by frequency multiplex, while with TDD modes data is transmitted from uplink and downlink connections on the same frequency by time multiplex.
A base station is a central unit in a communication network, which, in the case of a cellular mobile radio network, controls communication terminals within a cell of the mobile radio network via one or more radio channels. The base station provides the air interface between base station and terminal, and it is responsible for radio operation with the mobile subscribers and monitors the physical radio connection. It also transmits useful and status messages to the terminals. The base station has no switching function just a service function. A base station has at least one transmit/receive unit.
A terminal can be any communication terminal, through which a user communicates in a communication system. It can for example be a mobile radio terminal or a portable computer with a radio module. A terminal is often also referred to as a mobile station (MS) or in UMTS user equipment (UE).
In mobile radio, a distinction is made between two connection directions. The downlink (DL) refers to the transmission direction from the base station to the terminal. The uplink (UL) refers to the counter transmission direction from the terminal to the base station.
In broadband transmission systems, e.g. a UMTS mobile radio network, a channel is a sub-area of the overall transmission capacity available. In the context of this application a wireless communication path is referred to as a radio channel.
In a mobile radio system, e.g. UMTS, there are two types of physical channel for data transmission: permanently assigned or dedicated channels and shared-used or common channels. In the case of dedicated channels, a physical resource is reserved solely for the transmission of information for a specific terminal. In the case of common channels information can be transmitted, which is intended for all terminals, for example the primary common control physical channel (P-CCPCH) in the downlink, or all the terminals share a physical resource, with each terminal only being able to use it for a short time. This is the case for example with the physical random access channel (PRACH) in the uplink.
During transmission via a common or dedicated channel, the data undergoes both bandwidth spreading by means of a spread code or channelization code for more reliable transmission and also a scrambling procedure to identify a specific connection. Different types of scrambling code are used for this purpose as a function of the transmission direction, channel type and radio transmission technology.
While a bit from a data sequence is generally referred to as a symbol, a bit from a bandwidth-spread sequence is referred to as a chip.
In mobile radio systems such as UMTS packet switched services, by means of which data is transported in packets, are provided as well as circuit switched services.
The so-called high-speed downlink shared channel (HSDSCH), to which a corresponding control channel, for example the shared control channel for HS-DSCH (HS-SCCH), is assigned represents an extension of the existing downlink-shared channel (DSCH).
Determination of Channel Quality in the Case of the HSDPA in UMTS
Until now, to allow the transmitter (e.g. the base station) to know the channel quality at the data receiver, a message or channel test message was sent to the data transmitter, which the transmitter could use to estimate the channel quality with which data is received at the receiver. In the case of a downlink data transmission for the HSDPA in the UMTS system, this takes place as follows: the mobile station sends a standardized message or CQI (channel quality indicator) message to the base station. This CQI message contains information about the channel quality tested by the receiver in a predefined, standardized form. The base station can use this to determine the channel quality with which data is received at the receiver. The base station selects data transmission parameters for the transmission of data to the mobile station based on the channel quality determined. These data transmission parameters could for example be the modulation system, the coding rate or the transmit power.
However, channel quality can change over time. Therefore until now a CQI message was sent at regular intervals from the mobile station to the base station, so that the channel quality could be repeatedly determined and notification thereof given.
In summary, transmission of the CQI message resolves the problem of channel quality having to be known in the transmitter. However the CQI message also has to be transmitted and therefore takes up resources in the uplink, i.e. in transmission from the mobile station to the base station. To keep these aforementioned resource burden as low as possible, and also to obtain channel information with adequate accuracy, the following steps have been taken:
1) The CQI message is not transmitted in every frame, i.e. with maximum possible frequency, but only once in k frames, the mobile station being informed of k by the base station. For example transmission characteristics do not change very quickly at low speed and a lower CQI message transmission rate is adequate. However a higher transmission rate (i.e. smaller k) is required when mobile stations are moving quickly, with the channel changing very significantly over time.
2) If a CQI message is transmitted when data decoding has failed in the receiver. The base station is notified of this with a so-called NACK (Negative ACKnowledge) message. Further to this NACK, according to this proposal the mobile station should send a CQI, on the basis of which the base station can make a better adjustment in the future.
However this method only resolves the problem to an unsatisfactory degree, as shown below:
If the transmission quality is too good rather than too poor, all messages are typically received correctly and no NACKs are transmitted only ACKs (Positive ACKnowledge or positive confirmation). However, transmission with too good of a quality is also sub-optimal, as it needlessly takes up resources (in particular base station transmit power or additional interference in all other mobile stations), which are not really necessary and which could therefore be more usefully used for other connections.
The mode of operation of an ARQ method (in particular HARQ hybrid ARQ) is also not optimal, if all or almost all packets are received correctly straight away but decoding fails in 10-30% of cases. In such cases retransmission must be requested, which means an increased resource outlay, but on the other hand transmission can take place at a significantly lower power, if it only has to be correctly decoded with a probability of 70 to 90%, than if a higher decoding value were required. Energy and resources are therefore generally saved.
However this arrangement means that even with optimum adjustment a CQI message is still transmitted in 10 to 30% of cases, even if it is not necessary, because the adjustment is already optimal and therefore does not have to be changed.
According to a further proposal the current channel status is not transmitted, at least at high speeds, but the history averaged over several frames. The reason for this is the fact that the channel characteristics due to fast fading change so quickly at high speeds that the information about them is already out of date when it arrives at the base station. The general position of a mobile station and the additional channel attenuation produced by shadowing and diffraction phenomena at large structures, so-called log-normal fading, however change much more slowly and are less quickly out of date. Averaging eliminates the fluctuations due to fast fading and allows a more accurate mean value to be determined. This method does not resolve any problems relating to optimum fast transmission per se but it at least improves knowledge of the mean transmission quality.